In a case where a press, a machine tool, a construction machine, a ship, an aircraft, an unmanned conveying unit, an unmanned warehouse or the like installation are centrally controlled, a large number of sensors (limit switch, actuating button, encoder or the like) for detecting an operative state of each unit in a system and actuators (valve, relay, lamp or the like) for controlling the operative state of each unit in the system are required. In a case of, e.g., the press, the number of sensors and actuators, as mentioned above, exceeds, 3,000. With respect to some other unit, it is expected that much more than 3,000 sensors and actuators will be required.
Hitherto, a generally available central control system for centrally controlling the aforementioned kind of units is typically constructed such that a number of sensors and a number of actuators are connected directly or via relaying means to a machine controller in parallel with each other, outputs from the sensors are received in the machine controller, and the actuators are driven under proper control in response to signals from the machine controller.
In a case of the conventional central control system as mentioned above, as the number of sensors and the number of actuators become very large, the number of connecting lines for connecting the machine controller to the sensors and the actuators, as well as the total length of connecting lines extending therebetween, not only become also very large but also erroneous line connection may take place. In addition, an input/output section in the machine controller becomes very complicated in structure.
To obviate the foregoing problems, a proposal has been made as to an arrangement in which a plurality of nodes are connected to each other in series, one or a plurality of sensors and one or a plurality of actuators are connected to each node, these plural nodes are connected to one after another via a main controller in a loop-shaped configuration or in a daisychain-shaped configuration and each node is controlled in response to signals from the main controller. With such an arrangement, basically, the main controller requires only signal input lines and signal output lines connected thereto. Similarly, each node requires only signal input lines and signal output lines connected thereto. Consequently, the number of connection lines to be connected to the main controller and the time required for a wiring operation can be reduced remarkably.
A conventional technique of making serial connection, as described above, is disclosed in, e.g., an official gazette of Japanese Laid-Open Patent No. 118048/1986. According to this conventional technique, a date frame signal is composed of a synchronous slot SYN, data slots DFa to DFe for five nodes a to e, an end slot E or the like, and a data length can be fixed by unchangeably allocating the date slots DFa to DFe to the five nodes, as shown in FIG. 1. Specifically, each node is constructed such that control data DO corresponding to a plurality of actuators at the node are extracted from the data region DF corresponding to the node in an inputted data frame signal and detection data Di from a plurality of sensors at the node are introduced into the data region DF. Here, a processing to be taken for a node b will be described as an example in detail in the following. Control data DOb on own node in the data frame signal are once stored in storing means for the node b and thereafter they are fed to a plurality of actuators. Further, control data DOc, DOd and DOe on nodes subsequent to the own node are stored in other storing means and then they are additionally placed on locations behind detection data Dib derived from a plurality of relevant nodes, whereby a frame signal to be sent to a node c at the subsequent stage is created.
With the conventional technique as described above, however, when simultaneity is taken into account with respect to receiving of outputs from respective sensors connected to a plurality of node and feeding of data to a plurality of actuators connected to the nodes, the simultaneity can be realized on the basis of node in unit but it can not be assured between or among plural nodes. Namely, according to the conventional technique, after an inputted data frame signal is once stored in storing means and input/output data are then introduced/extracted, the frame signal is sent to a node at the subsequent stage. Thus, as shown in FIG. 2, a time when the data frame signals are inputted into respective nodes is deviated from a next time at least by one frame with the result that it is entirely impossible to assure the above-described simultaneity. If an arrangement is made with the conventional apparatus such that introduction of the detection data Di of the sensors into the storing means at each node and extraction of the driving data DO to be fed to the actuators from the storing means are accomplished at a same time, the simultaneity can be maintained. To this end, however, special arrangement should be made for correct time control in common with all the nodes but there arises another problem that a period of sending such data frame signal should be controlled strictly.
Further, since the conventional apparatus is constructed in such a manner that the data slots DFa to DFe of the data frame signal are unchangeably allocated to the five nodes, a position where input/output data are introduced/extracted in the data frame signal at each node is different from node to node whereby each node cannot entirely constructed with a same circuit structure. This leads to a result that the apparatus is fabricated at an increased cost.
With respect to the simultaneity of receiving data from sensors and feeding data to actuators to be driven, there occur the following malfunctions, if the simultaneity is not maintained across all the nodes.
For example, a detection value of each sensor is not limited only to one bit but in some case it has eight or more bits. However, data from the sensors each including an output line having plural bits can not necessarily be inputted into a single node but there may arise an occasion that outputs from a single sensor should be inputted across two or more nodes in dependence on limitation on the number of input data relative to a node or the like factor. In such a case, if the simultaneity of receiving data is not maintained across the respective nodes, the result is that completely incorrect data are received when sensor data vary for a period of time when data are received with deviation across the respective nodes.
With respect to actuators, there arise the following problems in addition to the aforementioned problems inherent to sensors.
In a case where a machine is provided with two or more actuators, such as clutches for controlling power, valves for controlling a flow rate or the like as actuators for driving the machine, the actuators are not necessarily controlled by a single node. Usually, they are controlled across two or more nodes. Also in such a case, if the simultaneity of feeding data across the respective nodes is not maintained, it is impossible to control the machine by simultaneously changing operative state of each of two or more actuators.
In this manner, with the system including a plurality of nodes connected to each other in series, to maintain the simultaneity of receiving data and feeding data across the respective nodes is an especially important subject from the viewpoint of assuring improved reliability of the system and extended running life of associated units and components.
The present invention has been made with the foregoing background in mind and its object resides in providing an apparatus for carrying out serial control which assures the simultaneity of receiving data and feeding data across respective nodes under a condition that all the nodes are designed and constructed with the same circuit structure.